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Energies 2019, 12, 809 9 of 41 Decarboxylation and decarbonylation are commonly referred as deCOx reactions. In decarboxylation the O2 molecules are removed in the form of CO2 while in decarbonylation the oxygen molecules are removed as CO and H2O. In both these deCOx cases the saturated hydrocarbon produced has one C atom less than the parent fatty acid chain in the triglyceride. In HDO the oxygen molecules are removed exclusively as H2O and the saturated hydrocarbon has an equal number of C atoms with the corresponding fatty acid bound in the triglyceride. As a result, the saturated hydrocarbons produced by the hydro-processing process will have about the same C atoms with the fatty acid chains of the triglycerides. The three SDO reactions form a complex mechanism in which the accurate determination of their individual contribution is generally difficult. However, the catalytic preference to deCOx or HDO reactions may be estimated by the distribution of the liquid hydrocarbons and the C17/C18 ratio and the preference between the deCO2 and deCO reactions may be found through the CO2/CO ratio [55,57]. The conversion of the triglycerides, the degree of deoxygenation (DOD) and the yield of normal saturated hydrocarbons tend to increase with the reaction temperature in both batch and continuous reactors. However, the yield of C15–C18 hydrocarbons maximizes at an optimal temperature and then reduces due to hydrocracking and reverse water gas shift (RWGS) reactions. Higher H2 pressures have been observed to enhance both the yield of hydrocarbons and the selectivity to green diesel. Also, the higher H2 pressures promote the HDO reaction pathway [39]. The H2 consumption obviously depends on the chemistry of the feedstock. Highly unsaturated oils such as rapeseed oil and fish oils require higher H2 consumption since they have more double bonds. Finally, in all cases, propane (C3H8) is produced as a side product together with H2O, CO and CO2 from the SDO reactions. The deoxygenation of triglycerides provides a green diesel consisting mainly of normal (straight chain) saturated hydrocarbons in the C15–C18 range. These hydrocarbons have a high cetane number but poor cold flow properties since they have a high freezing point above 15 ◦C [58]. One solution to this problem is to blend green diesel with petroleum diesel as a cetane number improver. Another solution to this problem comes with the hydroisomerization of the normal saturated hydrocarbons into branched chain isomers with lower freezing point appropriate for use in engines under cold weather and cold start conditions, though with the cost of lower cetane numbers [55,56]. Isomerization changes the structure of a molecule leaving its molecular weight the same. Given that saturated hydrocarbons are not prone to direct isomerization they must be processed in an appropriate way and hydroisomerization is an appropriate technique in this direction. The hydroisomerization of a saturated hydrocarbon takes place in three steps starting with its dehydrogenation into the same carbon atom alkene and proceeds with the skeletal isomerization of the alkene and its final hydrogenation into the branched isomer saturated hydrocarbon. This three step process is feasible in light saturated hydrocarbons with less than 7 carbon atoms but it is generally difficult for heavier molecules without some extent of hydrocracking [59]. In the case of the heavy saturated hydrocarbons of the green diesel, isomerization needs a balanced action of hydroisomerization and hydrocracking over an appropriate heterogeneous catalyst that can increase selectivity to the desired isomer hydrocarbons [60,61]. This is achieved by using acidic catalysts such as commercial FCC catalysts, zeolites or other supports. The hydromerization of green diesel may lower the freezing point from 20 ◦C below −10◦ causing also a drop of the cetane number from about 100 to 70. The global market of green diesel is growing in great volumes, from 330 × 106 gallons in 2011 to 2.1 × 109 gallons in 2017 [62]. A list of the main green diesel producers is given in Table 4 [63]. Most of the producers have developed proprietary technologies (Neste NExBTL, UOP/Eni EcofiningTM, UPM BioVerno etc.) and standalone plants comprised by the biomass cleanup and pretreatment section, the deoxygenation (hydrotreatment) section, a hydroisemarization reactor and a separation column, as shown in Figure 3a. Given that green diesel can be mixed with conventional petroleum diesel to satisfy the automotive fuel specifications, some oil refineries have developed methods for the simultaneous co-processing of triglyceride feedstocks with petroleum intermediates such as straight run gas oil and/or vacuum gas oil, as shown in Figure 3b [55,56]. This is economicallyPDF Image | Green Diesel: Biomass Feedstocks, Production Technologies
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